1. Technical Field
The present invention generally relates to transformers and in particular to bushings utilized within transformers. Still more particularly, the present invention relates to a method and apparatus for increasing the rating of a transformer via enhancements to the bushing utilized within the transformer.
2. Description of the Related Art
In the oil-filled transformer industry, transformer capacity is determined based on how much voltage the transformer can safely handle. The amount of voltage a transformer can handle depends on several factors, including the electrical connections of the bushing utilized within the transformer, i.e., the bushing terminals. Bushing terminals are used to hold the electrical conductors and transfer current from outside of the transformer to windings or coils on the inside of the transformer without shorting out on the transformer wall.
Bushing terminals (or bushings) commonly comprise an insulating bracket, a conductor, and provision for mounting the bracket to the transformer to insulate the conductor from the transformer wall. Bushings are rated on how well they insulate the transformer wall from current flowing through the conductor. One factor that must be considered in the rating of bushing terminals is the presence of contaminants on the bushing. Contaminants on the bushing provide a conductive path, which can cause electricity to leak out onto the transformer wall and cause electrical arcing or striking. Arcing occurs when the difference in potential between the transformer wall and the conductor becomes sufficiently large. The air ionizes between the transformer wall and the conductor, and this creates a path of relatively low resistance through which current can flow. The resulting blast of electricity can cause a short circuit to occur and severely damage the transformer.
Another factor used to rate bushings is electrical xe2x80x9ccreepagexe2x80x9d across the insulating bracket. Creepage is the electrical leakage on a solid dielectric surface. Creepage distance is the shortest distance on a dielectric surface between two conductive elements. The current will essentially track or crawl across the insulating bracket onto the transformer wall. The onset of creepage can produce similar effects to contamination in that a short circuit to the transformer wall can occur and cause damage to the transformer itself.
One solution utilized to minimize these problems is the use of a material with a high insulating property such as porcelain to create the bushing. One such porcelain bushing is manufactured by Normandy Machine Co. Inc. of Troy, Mo. and is rated to 5 kV (5000 volts). However, porcelain is not durable and is easily broken. A broken or damaged bushing requires costly replacement and down time of the transformer. To make the insulating material more durable, plastic is often used. One such plastic bushing is manufactured by Central Moloney Inc of Pine Bluff, Ark. under the trademark of Tuf Ex-Mount(trademark).
FIG. 4A is a line drawing of a Central Moloney Inc. Tuf Ex-Mount(trademark) molded transformer bushing as cited above. Bushing 400 contains insulating mounting bracket 408 having a first shoulder 410 and second shoulder 406. Extending from second shoulder 406 of insulating mounting bracket 408 is a threaded portion 402 of a conductor 404. Conductor 404 extends from first shoulder 410 of insulating mounting bracket 408 and includes a front end 412 utilized for connecting to transformer windings. FIG. 4B represents bushing 400 installed in an oil-filled transformer. Typically, bushing 400 is not immersed in oil 414 causing a reduction in voltage capacity.
When mounted, the inside and outside surfaces of Tuf Ex-Mount(trademark) are rated at 1.2 kV. In addition, the creepage and strike distances are also designed for use at 1.2 kV. Notably, the molded 1.2 kV bushing is designed and used such that the external hardware may be installed or exchanged in the field. The low voltage rating of the plastic bushing when compared to the porcelain bushing is primarily due to its smaller dimensions. Utilization of 1.2 kV Tuf Ex-Mount bushing at a higher voltage than 1.2 kV will result in higher incidents of arcing, striking, or creepage across the insulating bracket.
Therefore, the present invention recognizes that a need exists for a durable, cost effective method for reducing the external creepage and incidents of arcing when using transformer bushings.
It is therefore one object of the present invention to increase the voltage class of a transformer.
It is another object of the present invention to decrease the external creepage on bushings.
It is yet another object of the present invention to provide an increased strike distance from the conductor of the bushing to a transformer wall.
The foregoing objects are achieved as is now described. The voltage class rating of a transformer is increased by increasing the rating of an off the shelf molded transformer bushing. Specifically, the voltage class rating is increased by increasing the distance between the non-insulated portion of a conductor of a transformer bushing and the transformer wall. Insulating material is connected to the external insulating shoulder of the bushing and covers some of the external portion of the conductor. In the preferred embodiment, the insulating material comprises heat shrink tubing, and, in one variation of the preferred embodiment, mastic is applied beneath the insulating material.
When the modified transformer bushing is installed into a transformer, an internal end of the conductor is made to extend below the surface of the oil in the transformer. The level of the oil also covers an internal insulating shoulder of the bushing. The insulating values of the mastic and heat shrink tubing and the insulating value of the conductor immersed in oil combine to increase the voltage class rating of the off the shelf molded transformer bushing and thereby the rating of the transformer with a nominal increase in costs.
In one embodiment, a conductive extension is connected to the conductor of the bushing. The insulator is then extended beyond the conductor to also cover a portion of the extension. This provides an even longer distance between the exposed conductive end and the transformer wall, further reducing the occurrence and effects of arcing and electrical creepage.
The above as well as additional objectives, features, and advantages of the present invention will become apparent in the following detailed written description.